This invention relates to the field of medical devices, and more particularly to a method of forming a thin-walled member for an intralumenal medical device.
In the design of intralumenal catheters, the progression of improvements has been to decrease the catheter profile, while accommodating the conflicting characteristics of catheter strength and flexibility. These catheter characteristics significantly effect catheter maneuverability and, therefore, the ability to effectively position the catheter at a desired location within a patient. The catheter profile and flexibility are a function of the wall thickness of the various catheter components. While the catheter shaft must have sufficient strength, and therefore sufficient wall thickness, to provide torque transmission and kink resistance, sleeves disposed about the catheter balloon or shaft are generally made very thin. Such sleeves are used to strengthen, protect, or otherwise modify the surface of the catheter balloon or shaft. For example, a sleeve may be provided at the junction of two catheter shaft sections to strengthen the joint. Also, balloon catheters used for stent delivery are typically provided with a protective sleeve between the balloon and the stent mounted thereon. The wall thickness of such tubular sleeves should be very thin to minimize the effects on catheter profile and flexibility.
Polymeric tubular catheter components are typically formed by extrusion, although they may be made by a variety of methods depending on the material used and the desired characteristics of the component. For example, in free extrusion, melted polymeric material passes through an extrusion die over a mandrel. Where a multilayered article is desired, the layers may be co-extruded, or a second layer extruded over an existing polymeric tube. The wall thickness of an extruded tube is a function of the annular gap between the die and the mandrel, and post extrusion processing such as draw-down of the polymeric material. Draw-down is the ratio of the die diameter to the final diameter of the extruded article, and is therefore a measure of the thinning of the extruded article as it exits the extruder.
While extrusion is a preferred technique for forming polymeric tubular catheter components, a thin-walled continuous as-extruded tube has heretofore been unavailable. A variety of technical problems are encountered which prevent the extrusion of a tube having a wall thickness of about 0.003 inch (0.0076 cm) or less, that is suitable for use as a catheter component. For example, such thin-walled tubes loose shape retention during movement through the solidification phase and the take-up system of the extruder. Because of the small annular gap between the die and mandrel used to produce the thin-walled article, the extruded article is prone to melt fracture, i.e. the formation of a rough and irregular surface on the article. Moreover, a tube produced by extrusion using a conventionally sized die and mandrel annular gap cannot be processed into a thin-walled catheter component by drawing-down the tube, because the degree to which wall thickness can be thus reduced is limited. For example, during draw-down to produce a thin-walled tube, the tube wall may tear, or intralumenal air pressure used to maintain the inner diameter dimensions of the tube will produce ballooning of the tube wall.
Therefore, what has been needed is a method of producing a thin-walled tubular catheter component by extrusion. The present invention satisfies these and other needs.